Chickweed and henbit weeds are bad in alfalfa this fall. DNBP (premerge or Sinox PE) sprayed on infested fields at rate of 1 to 3 qt in 20 to 40 gallons of water per acre may destroy the weeds without harm to the dormant alfalfa. Use the higher rate if chickweed is matted on the ground or if henbit is heavy in the stand. Spray only when the temperature is 55 to 60 degrees or warmer and fair weather is predicted for 12 hours after treatment. The 1-qt rate applied in early fall when chickweed is very small (not until August seeded alfalfa plants have 2 or more trifoliate leaves) and repeated during a warm spell in March should give good control of chickweed and germinating henbit.
Freeman, J. F., "Controlling Chickweed and Henbit in Alfalfa" (1965). Agronomy Notes. 204.